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Date: Fri, 15 Mar 2024 06:44:56 -0700
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Subject: Re: [PATCH] timer/migration: Remove buggy early return on
 deactivation [was Re: Unexplained long boot delays [Was Re: [GIT PULL] RCU
 changes for v6.9]]
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To: Frederic Weisbecker <frederic@kernel.org>,
 Boqun Feng <boqun.feng@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
 "Russell King (Oracle)" <linux@armlinux.org.uk>,
 Joel Fernandes <joel@joelfernandes.org>,
 Anna-Maria Behnsen <anna-maria@linutronix.de>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 linux-kernel@vger.kernel.org, kernel-team@meta.com, paulmck@kernel.org,
 mingo@kernel.org, rcu@vger.kernel.org, neeraj.upadhyay@amd.com,
 urezki@gmail.com, qiang.zhang1211@gmail.com, bigeasy@linutronix.de,
 chenzhongjin@huawei.com, yangjihong1@huawei.com, rostedt@goodmis.org,
 Justin Chen <justin.chen@broadcom.com>
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 <ZfNHNvzpqf8DOZd8@boqun-archlinux>
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From: Florian Fainelli <f.fainelli@gmail.com>
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On 3/14/2024 6:14 PM, Frederic Weisbecker wrote:
> On Thu, Mar 14, 2024 at 03:05:53PM -0700, Boqun Feng wrote:
>> I notice CPU3 didn't have its own non-deferrable timer queued (local or
>> global), so could the following happen?
>>
>> 	timer_base_try_to_set_idle():
>> 	  __get_next_timer_interrupt():
>> 	    fetch_next_timer_interrupt():
>> 	      // nextevt_local == nextevt_global == basej + NEXT_TIMER_MAX_DELTA
>> 	      // tevt->local == tevt->gloabl = KTIME_MAX
>> 	    timer_use_tmigr():
>> 	      tmigr_cpu_deactivate():
>> 	        __tmigr_cpu_deactivate():
>> 		  // tmc->cpuevt.ignore untouched still == true
>> 		  walk_groups(&tmigr_inactive_up, ...):
>> 		    tmigr_inactive_up():
>> 		      data->remote = true;
>> 		      tmigr_update_events():
>> 		        if (child) { // child is NULL
>> 			  ...
>> 			} else {
>> 			  first_childevt = evt = data->evt;
>>
>> 			  if (evt->ignore && !remote)
>> 			    return true; // no remote tick is picked.
>> 			  ...
>> 			}
> 
> Nice catch! Florian can you try the following?
> 
>  From b0e335371ed758f68bf4f501246298c98a615b04 Mon Sep 17 00:00:00 2001
> From: Frederic Weisbecker <frederic@kernel.org>
> Date: Fri, 15 Mar 2024 00:21:01 +0100
> Subject: [PATCH] timer/migration: Remove buggy early return on deactivation
> 
> When a CPU enters into idle and deactivates itself from the timer
> migration hierarchy without any global timer of its own to propagate,
> the group event of that CPU is set to "ignore" and tmigr_update_events()
> accordingly performs an early return without considering timers queued
> by other CPUs.
> 
> If the hierarchy has a single level, and the CPU is the last one to
> enter idle, it will ignore others' global timers, as in the following
> layout:
> 
>             [GRP0:0]
>           migrator = 0
>           active   = 0
>           nextevt  = T0i
>            /         \
>           0           1
>        active (T0i)  idle (T1)
> 
> 0) CPU 0 is active thus its event is ignored (the letter 'i') and so are
> upper levels' events. CPU 1 is idle and has the timer T1 enqueued.
> 
>             [GRP0:0]
>           migrator = NONE
>           active   = NONE
>           nextevt  = T0i
>            /         \
>           0           1
>        idle (T0i)  idle (T1)
> 
> 1) CPU 0 goes idle without global event queued. Therefore KTIME_MAX is
> pushed as its next expiry and its own event kept as "ignore". As a result
> tmigr_update_events() ignores T1 and CPU 0 goes to idle with T1
> unhandled.
> 
> This isn't proper to single level hierarchy though. A similar issue,
> although slightly different, may arise on multi-level:
> 
>                              [GRP1:0]
>                           migrator = GRP0:0
>                           active   = GRP0:0
>                           nextevt  = T0:0i, T0:1
>                           /              \
>                [GRP0:0]                  [GRP0:1]
>             migrator = 0              migrator = NONE
>             active   = 0              active   = NONE
>             nextevt  = T0i            nextevt  = T2
>             /         \                /         \
>            0 (T0i)     1 (T1)         2 (T2)      3
>          idle         idle            idle         idle
> 
> 0) CPU 0 is active thus its event is ignored (the letter 'i') and so are
> upper levels' events. CPU 1 is idle and has the timer T1 enqueued.
> CPU 2 also has a timer. The expiry order is T0 (ignored) < T1 < T2
> 
>                              [GRP1:0]
>                           migrator = GRP0:0
>                           active   = GRP0:0
>                           nextevt  = T0:0i, T0:1
>                           /              \
>                [GRP0:0]                  [GRP0:1]
>             migrator = NONE           migrator = NONE
>             active   = NONE           active   = NONE
>             nextevt  = T0i            nextevt  = T2
>             /         \                /         \
>            0 (T0i)     1 (T1)         2 (T2)      3
>          idle         idle            idle         idle
> 
> 1) CPU 0 goes idle without global event queued. Therefore KTIME_MAX is
> pushed as its next expiry and its own event kept as "ignore". As a result
> tmigr_update_events() ignores T1. The change only propagated up to 1st
> level so far.
> 
>                              [GRP1:0]
>                           migrator = NONE
>                           active   = NONE
>                           nextevt  = T0:1
>                           /              \
>                [GRP0:0]                  [GRP0:1]
>             migrator = NONE           migrator = NONE
>             active   = NONE           active   = NONE
>             nextevt  = T0i            nextevt  = T2
>             /         \                /         \
>            0 (T0i)     1 (T1)         2 (T2)      3
>          idle         idle            idle         idle
> 
> 2) The change now propagates up to the top. tmigr_update_events() finds
> that the child event is ignored and thus removes it. The top level next
> event is now T2 which is returned to CPU 0 as its next effective expiry
> to take account for as the global idle migrator. However T1 has been
> ignored along the way, leaving it unhandled.
> 
> Fix those issues with removing the buggy related early return. Ignored
> child events must not prevent from evaluating the other events within
> the same group.
> 
> Reported-by: Boqun Feng <boqun.feng@gmail.com>
> Reported-by: Florian Fainelli <f.fainelli@gmail.com>

Gave it a good 20 cold boots and suspend to DRAM cycles and none of 
those triggered the reported behavior:

Reported-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>

Thanks for the quick fix Frederic!
-- 
Florian